The present invention relates generally to a method for fabricating a master recording disc to be used for mass-producing digital audio or video discs.
Master recording discs for digital audio and video disc recording are formed with pits of tapered side walls. The tapered-wall pits are advantageous since they ease the separation of molded plastic discs from the mold and are further advantageous for tracking purposes. The concept of this tapering is shown and described in Japanese Laid-open Patent Publication No. 55-55449. However, it has been difficult to form tapered-wall pits on a glass substrate of the master recording disc.
One approach that is shown and described in Japanese Laid-open Patent Publication No. 55-101144 involves the use of a master blank composed of glass substrate, a layer of SiO.sub.2 deposited thereon and a photoresist layer on the SiO.sub.2 layer. An intensity-modulated laser beam is directed from the master blank to the photoresist layer to produce a series of optically excited regions in track turns. The master blank is subsequently developed to form a series of pits which extend down to the boundary surface of the photoresist and SiO.sub.2 layers, and then is exposed to the plasma of CF.sub.4 gas in a vacuum chamber. The SiO.sub.2 layer is etched by the plasma etchant so that the pits are enlarged at the top and tapered downwardly to the boundary surface of the SiO.sub.2 layer and the glass substrate. Although tapered-wall pits are produced in the SiO.sub.2 layer, it is difficult to maintain a consistent taper angle.